The story you tell yourself creates your reality.
Part of the activation energy required to start any task comes from the picture you get in your head when you imagine doing it. It may not be that going for a run is actually costly; but if it feels costly, if the picture in your head looks like a slog, then you will need a bigger expenditure of will to lace up.
Slowness seems to make a special contribution to this picture in our heads. Time is especially valuable. So as we learn that a task is slow, an especial cost accrues to it. Whenever we think of doing the task again, we see how expensive it is, and bail.
That’s why speed matters.